Human binocular vision is essentially determined by the motility of the eye, the fixation ability, the convergence and fusion ability, the accommodation ability and the ability of stereoscopic viewing. The basic functions are interconnected and partly interdependent through physiological and neurological processes.
The fixation ability determines how much each eye can be oriented to a fixation point, such as an object, and follow a moving object. Thanks to a neurological control loop, the eye is turned via the eye musculature so that the image of the fixation point on the retina comes to lie constantly on the fovea centralis.
The convergence ability depends on how much both eyes can be oriented to the same fixation point during binocular vision. Normally, the directions of viewing of both eyes when looking at an infinitely remote object are parallel. At shorter distance from the object, a convergence angle results between the two directions of viewing, depending on the distance of the object. Thanks to neurological control loops, the deformation of the lens is regulated by the ciliary muscle so that the fixation point is imaged sharply on the fovea centralis.
Fusion is a neurological process in which the images of the two eyes are combined in the brain into a single overall image. The result is a “3D impression”. This process plays out in the brain and cannot be observed from the outside. The ability of spatial or stereoscopic vision (stereopsis) is based on the ability of the brain to ascertain depth information from the differences between the two images on the basis of the adjusted convergence angle.
The accommodation ability determines the degree to which the refractive power of the lens can be changed by means of the ciliary muscle and adapted to the particular object viewing distance, so that objects from different distance can be imaged sharply on the retina. In the relaxed state of the eye in persons with normal vision, far removed objects are sharply imaged on the retina.
Accommodative convergence means the orientation of both eyes on the same fixation point. For close objects, the eyes are turned toward the nose, so that the two lines of sight approach each other (converge) and finally intersect at the object. The angle of convergence at which the lines of sight intersect is greater as the distance is less between observer and object.
Two neurophysiological processes determine the accommodation ability of the eyes. One is based on the autofocus principle, which is also used in cameras. The focus of the eye is slightly varied and a check made to determine if the image acuity has improved or grown worse. Depending on the result, the focus is restored or the adjustment is continued until an optimum is found. This process enables a focusing of the eye during monocular vision on objects at different distance. In binocular vision, a second neurophysiological process is at work, known as near focus strias. Accommodative convergence and accommodation stand in a fixed relation to each other, described by the so-called AC/A quotient. The near focus strias always “strives” to adjust this constant optimal value. If the angle of convergence is known, the distance of the viewed object can be determined from this, and also the required accommodation from the distance. This relationship is used in the brain by the neurophysiological coupling of convergence and accommodation.
In similar fashion, the CA/C quotient characterizes the ratio of convergence accommodation (CA) to convergence (C).
Patent WO 2011/107244 A1 specifies a device for subjective refraction determination, in which several objects with different focal positions are generated and shown to the test subject for comparison. In the binocular version, however, the convergence of objects in different focal positions which is necessary for natural vision is not taken into account. The two comparison symbols are thus not represented realistically. A disturbance of the near focus strias and a falsification of the subjective refraction are therefore possible.
A device and a method for determining the subjective refraction in binocular vision are also specified in patent US 2008/0246921 A1. While this device enables a stimulation of accommodation, vision test charts are used, which cannot produce any stimulus for convergence. Neither is there any viewing direction measuring apparatus for measuring the position of the eye. Thus, a correct stimulus for the near focus strias cannot be guaranteed. Yet the correct stimulation of the near focus strias as a binocular neurological control loop can be of decisive significance for just such a subjective binocular refraction measurement.
In patent DE 195 01 415 C2 an open view apparatus is described, in which virtual test objects can be displayed in the real environment. This is a binocular apparatus, which correctly simulated accommodation and convergence. However, accommodation and convergence are firmly coupled, so that the AC/A quotient is not variable. This is a problem, because the individual AC/A value fluctuates from one person to another on account of different eye spacings and other factors. Thus, one must expect a disturbance of the B near focus strias in persons with especially large or especially small eye spacing. Neither is it possible to simulate unnatural AC/A quotients, such as occur with virtual environments such as 3D movies or 3D television. Moreover, separate focusing for both eyes cannot be done. The accommodation cannot be measured with the apparatus, and the convergence can only be determined indirectly and subjectively.
U.S. Pat. No. 5,825,456 A specifies an apparatus for generating stereoscopic images in which convergence and size of the object are varied and compared to reference objects. It is supposed to investigate how a subject perceives relative and absolute distances subjectively. The device is in the form of spectacles and it generates a variable stereoscopic test character by means of displays. At the same time, eye-trackers (“gazing point detecting”) are used to determine the fixation points of both eyes. However, there is no variation of the focal position. Thus, the lack of an accommodation stimulus results in an unnatural AC/A value. Thus, an unnatural vision condition is produced, which disturbs the near focus strias. Hence, it is also likely that the subjective distance perception investigated in the patent is falsified as a result.
A publication on a clinical trial describes a “Shutter-Spectacle Haploscope with Eye Tracker Control for Purposes of Fusion Therapy”, see Klinische Monatsblätter Augenheilkunde 2003; 220: 629-633. With a shutter-spectacles and a computer monitor a stereoscopic image is produced. Using eye trackers, the fixation and convergence of the eyes are measured. There is a stimulus and a measurement for the convergence. A feedback control occurs by computer. The accommodation is not taken into account. Therefore, accommodation and convergence do not correspond, and a disturbance of the near focus strias is possible, which can lead to a falsification of the results of the study.
The above-described methods all have the problem that an unnatural AC/A quotient is produced during the simulation of a virtual three-dimensional environment or a vision test character. The AC/A quotient, i.e., the coupling of eye convergence and accommodation, is either not considered at all or is only correctly adjusted for persons with a particular eye spacing. This can cause a disturbance of the near focus strias and thus a falsification of the measurement results. Furthermore, either the convergence or the accommodation is investigated with the described methods/layouts.
DE 197 04 197 A1 specifies a monocular or binocular arrangement for the subjective refraction determination and other visual functions making use of test subject glasses provided with laser scanners. The arrangement is used subjective refraction determination. In the binocular variant of the arrangement, a beam path is provided for each eye to generate an image on the retina of the test subject, so that the images can be presented at the same time or in alternation. In the binocular visual impression, two images are merged into one.
JP 11 64 351 A specifies an ophthalmoscopic apparatus in which each eye is coordinated with an optical system, which can be adapted to the eye spacing as well as the points of fixation for the examination by means of adjustable carriers.
Many orthoptic studies today are performed manually by subjective methods. Due to the subjectivity, a comparability of the results of different examination apparatus is not possible without limitations. A direct linkage with EDP equipment and use of the associated capabilities of remote examination (such as telemedicine) are not given. Only “snapshot” measurements are possible with the aforementioned apparatus, measurements continuous over time are not possible.
Subjective methods only enable a limited number of measurements at relatively large intervals of time. A high-resolution time analysis of the accommodation process, for example, as compared to the stimulus, is not possible. Transient process of the convergence or the accommodation cannot be measured with the mentioned apparatus.
The mentioned apparatus cannot perform dynamic cover and uncover tests to determine tropia and phoria.